Generally, if alkali metal element or alkaline-earth-metal element is contained in the core of an optical fiber preform consisting of silica glass, the attenuation caused by Rayleigh scattering of an optical fiber will decrease because the viscosity of the core is reduced and the re-arrangement of glass is facilitated while the optical fiber preform is drawn into the optical fiber. Hereinafter, both of the alkali metal element and the alkaline-earth-metal element are called the “alkali metal group.”
If the core which contains an alkali metal group does not contain chlorine (or contains only low concentration of chlorine), the attenuation will be increased because glass defects occur as the connection in glass molecular structure is broken when the alkali metal group elements added to the central part of the core in the state of optical fiber preform spread during fiber-drawing. If the core contains sufficient quantity of chlorine, the attenuation caused by glass defects will decrease because the occurrence of glass defects is restrained as the chlorine connects with the alkali metal group.
In order to reduce the increase of attenuation caused by such glass defect, the core of optical fibers disclosed in JP 2009-541796A (Patent Document 1) and JP 2017-76053A (Patent Document 2) contains chlorine in addition to the alkali metal group. When the core is divided into inner core and outer core in the optical fiber disclosed by Patent Document 1, the chlorine concentration of the inner core is 0 to 100 mol ppm, and the chlorine concentration of the outer core is 500 mol ppm or more. In the optical fiber disclosed by Patent Document 2, the chlorine concentration of the inner core is 0 to 1000 mol ppm, and that of the outer core is 4000 mol ppm or more.
In either of Patent Documents 1 and 2, the chlorine concentration of the inner core is smaller than that of the outer core. Thus, it is thought that the increase of the attenuation due to glass defects can be restrained if the chlorine concentration of the inner core in which the alkali metal group is added is made lower to control crystallization while the chlorine concentration of the outer core is made higher to restore the glass defect caused during fiber-drawing.